Geological Unit - Lithostratigraphic Units

Lithostratigraphic Units

• Supergroup – two or more groups and lone formations
• Group – two or more formations
• Formation – primary unit of lithostratigraphy
• Member – named lithologic subdivision of a formation
• Bed – named distinctive layer in a member or formation
• Flow – smallest distinctive layer in a volcanic sequence

The component units of any higher rank unit in the hierarchy need not be everywhere the same.

Supergroup

The term "supergroup" may be used for several associated groups or for associated groups and formations with significant lithologic properties in common.

Group

A succession of two or more contiguous or associated formations with significant and diagnostic lithologic properties in common. Formations need not be aggregated into groups unless doing so provides a useful means of simplifying stratigraphic classification in certain regions or certain intervals. Thickness of a stratigraphic succession is not a valid reason for defining a unit as a group rather than a formation. The component formations of a group need not be everywhere the same.

Subgroup

Exceptionally, a group may be divided into subgroups.

Formation

Formations are the primary formal unit of lithostratigraphic classification. Formations are the only formal lithostratigraphic units into which the stratigraphic column everywhere should be divided completely on the basis of lithology. The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region and the detail needed for geologic mapping and to work out its geologic history. No formation is considered justifiable and useful that cannot be delineated at the scale of geologic mapping practiced in the region. There is no formal limit to how thick or thin a formation may be.

Member

The formal lithostratigraphic unit next in rank below a formation.

• It possesses lithologic properties distinguishing it from adjacent parts of the formation.
• No fixed standard is required for the extent and thickness of a member.
• A formation need not be divided into members unless a useful purpose is thus served.
• Formations may have only certain parts designated as members. A member may extend from one formation to another.

Bed

The smallest formal unit in the hierarchy of sedimentary lithostratigraphic units, e.g. a single stratum lithologically distinguishable from other layers above and below. Customarily only distinctive beds (key beds, marker beds) particularly useful for stratigraphic purposes are given proper names and considered formal lithostratigraphic units.

Flow

A discrete extrusive volcanic body distinguishable by texture, composition, or other objective criteria. The designation and naming of flows as formal lithostratigraphic units should be limited to those that are distinctive and widespread.